1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device wherein a III-V group nitride semiconductor film is epitaxial grown on a Si substrate.
2. Background Art
Epitaxial growth of a III-V group nitride semiconductor film, particularly AlxGayInzN (x+y+z=1, y≠0) film as an electronics/optoelectronics material has been proposed (see, for example, Japanese Patent Laid-Open No. 2005-243727). As a method of epitaxially growing this AlGaInN film, a metal organic chemical vapor deposition (MOCVD) is known.
Sapphire substrates have widely been used for epitaxial growth of GaN by MOCVD. However, sapphire substrates have a low degree of lattice matching with GaN and have a thermal expansion coefficient different from that of GaN, and attempts to improve the crystalinity of GaN were unsuccessful in the past. In recent years, after the establishment of a low-temperature buffer layer technique, the crystalinity of GaN has been improved and the development of device manufacturing techniques mainly for blue and white LEDs has advanced remarkably.
However, it is difficult to further improve the crystalinity in terms of dislocation reduction effect and so on. Sapphire itself has low heat conductivity, and heat cannot be sufficiently released from a device using sapphire, resulting in a deterioration in performance of the device. Therefore, hopes are being consistently put on other substrate materials. SiC having a high degree of lattice matching and stable even at a high temperature is one of candidates for use as a suitable substrate. With SiC, while its quality has been improved in recent years, there are problems that its price is high and it is difficult to increase the wafer diameter, as well as crystal quality problems such as micropipes.
In contrast, an effect of increasing the wafer diameter and an effect of reducing dislocations have been achieved for Si substrates, and Si substrates are steadily available at a low price. However, there is a lattice matching problem and a thermal expansion coefficient difference problem with Si substrates and GaN. In a case where a low-temperature AlN buffer layer and a GaN are successively grown on a sapphire substrate and the temperature of the substrate is thereafter returned to room temperature, compressive stress acts in the GaN layer on the sapphire substrate, so that a crack is not easily caused in the substrate. In a case where these layers are formed on a Si substrate in the same way, a tensile stress acts in the GaN layer and, therefore, a crack can easily be caused in the Si substrate. Further, there is, for example, a melt-back etching problem relating to the reaction between Ga (or GaN) and Si. In spite of many problems including those described above, a practical level of use of Si substrates has been reached with the advancement of techniques including multilayer buffer techniques in recent years.